Can HVAC systems help prevent transmission of COVID …

Advanced Electronics Practice

Can HVAC systems help prevent the transmission of COVID-19?

Modifications to heating, ventilation, and air-conditioning systems might help reduce the spread of the novel coronavirus by purifying air, improving ventilation, and managing airflows.

This article was written collaboratively by the Advanced Industries Practice. The authors include Stephanie Balgeman, Ben Meigs, Stephan Mohr, Arvid Niem?ller, and Paolo Spranzi.

July 2020

? 4FR/Getty Images

Although much remains unknown about COVID-19, scientists have established that the coronavirus is highly contagious and transmitted via air. Studies suggest that it primarily spreads when infected people cough, sneeze, or talk--actions that expel respiratory droplets containing particles of coronavirus in combination with mucus or saliva. If these droplets land on or are inhaled by others nearby, they could transmit the coronavirus.1 Touching doorknobs, computer screens, or other surfaces on which droplets have landed may also lead to infection. The now-common guidelines for wearing masks and physical distancing--typically, by remaining six feet away from others--reflect these observations.

before settling, smaller droplets may evaporate, leaving virus particles, referred to as aerosols, suspended in the air.2 A recent study demonstrated that coronavirus particles may be active for up to three hours after their release.3 Although the World Health Organization (WHO) initially held that the coronavirus could not be spread through aerosols, it recently reversed its stance. The WHO guidelines now state that airborne transmission of the coronavirus may be possible indoors, especially for people who spend extended periods in crowded, poorly ventilated rooms.4 The WHO's turnaround came after the organization received an open letter from 293 scientists asking the organization to reconsider its position on airborne transmission.5

Questions remain, however, about whether tiny coronavirus particles, of about 0.1 microns in size, can become airborne and travel greater distances. Although heavy droplets, of about five to ten microns, usually travel less than one meter

Given the concern about airborne transmission, building managers, safety experts, and others might take steps to optimize ventilation and airflow indoors and limit viral spread. Some simple moves may help (see sidebar "Low-tech strategies for

Low-tech strategies for preventing airborne viral transmission

In localities that have reopened, business leaders, school officials, and others have already taken many steps to make their facilities safer. Some have installed physical barriers, made corridors oneway, increased the frequency of cleaning, and widened the space between desks. Staggered shifts are now common at many companies to decrease physical distancing, and masks are often mandatory. Other simple steps that may prevent airborne transmission include the following:

-- rearranging furniture to avoid having several people on the same airflow "corridor"

-- opening windows in buildings with basic HVAC systems, which cannot filter or pull in outside air, to increase the exchange of fresh air

-- locking windows in buildings with central HVAC systems, when permissible, to reduce indoor temperature changes; if a fan does not have to increase its speed because of

an inflow of warm or cold air, turbulence will remain low

-- designing novel seating arrangements, such as having employees on a shop floor work back-to-back rather than face-to-face

-- limiting the number of people allowed in a room

-- replacing hand dryers with paper towels to reduce air turbulence

1 World Health Organization, "Q&A on Coronaviruses (COVID-19)," April 17, 2020, . 2 Michele Debczak, "Airborne vs. aerosol vs. droplet: What's the difference?" Mental Floss, April 14, 2020, . 3 Neeltje van Doremalen et al, "Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1," New England Journal of Medicine,

March 17, 2020, . 4 "Q&A: How is coronavirus transmitted?," World Health Organization, July 9, 2020, who.int. 5 Apoorva Mandavilli, "293 experts with one big claim: The coronavirus is airborne," New York Times, July 4, 2020, .

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Can HVAC systems help prevent the transmission of COVID-19?

preventing airborne viral transmission"). But this may also be a good time to think about improving air quality in buildings by significantly changing heating, ventilation, and air-conditioning (HVAC) systems or by making physical changes to manage indoor airflows.

Control-setting changes and upgrades to HVAC systems

HVAC systems6 can potentially spread a virus across rooms when high-speed air flows past an infected person to others, something that has been shown with Severe Acute Respiratory Syndrome in 2004.7 If airborne transmission is also possible with the coronavirus, a few control-setting changes and upgrades may help decrease the risk of spread through this route. If building managers take such actions, they might help their tenants feel more comfortable amid all the uncertainty about the coronavirus (see the sidebar "Earning the trust of tenants").

-- installing high-performance air-purification systems, as discussed in the next section

Options for air purification

Numerous technologies can purify air. Filtration is the most common and typically the most effective method for HVAC systems (Exhibit 1). Other technologies, including irradiation and thermal sterilization, inactivate biological particles in the air without removing them. HVAC systems can also incorporate ionic purifiers, ozone generators, and other devices for cleaning air.

Filters in residential or commercial HVAC systems are usually installed either at an air inlet or outlet or within the central air-handling unit. Since external air that flows into an HVAC system may be contaminated, especially in metropolitan areas where buildings are in close proximity, technicians sometimes install a pre-filter for incoming air.

One step that technicians could take involves configuring ducted HVAC systems to increase the rate of exchange with fresh fresh air from outside the building to reduce recirculation. Adjusting the settings may also help. Instead of shutting down overnight or on weekends, for instance, the HVAC system could run without interruption to increase the replacement of air and minimize airflow speeds.

In buildings with old or inflexible systems, technicians might consider upgrading HVAC hardware. Some of the most important might include these:

-- replacing fixed-speed fan motors with variablespeed ones to enhance the control of airflow and allow for a minimum setting that produces lower speed airflow

-- introducing sophisticated airflow-control systems, such as those that are sensitive to pressure, to allow for smoother adjustment of airflows

Earning the trust of tenants

Real-estate owners and building managers are obliged to protect the health and safety of their tenants. If evidence that COVID-19 can spread through airborne transmission mounts, they will need to act rapidly. Although more studies are needed about the airborne transmission of the coronavirus, owners and managers must still respect the concerns of their tenants and communicate with them directly about any planned or requested upgrades, including changes to HVAC systems. Frequent and transparent communication by company leaders is a crucial element of an effective COVID-19 return-to-work plan. The trust that such actions build will help owners and building managers create lasting relationships with tenants.

6 There are two main types of HVAC systems. Ductless ones heat and cool air directly in rooms. Systems with a central duct can move outside air into buildings. This section focuses on systems with central ducts.

7 Ignatius TS et al, "Evidence of airborne transmission of the severe acute respiratory syndrome virus," New England Journal of Medicine, April 22, 2004.

Can HVAC systems help prevent the transmission of COVID-19?

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Filtration is the most common air- ltration method, followed by irradiation

Filtration

Irradiation

Thermal

Other

Air is forced through ber-based material or membrane

E cacy varies, depending on lter type

Uses electromagnetic radiation, most commonly ultraviolet-C light, to deactivate pathogens

Cannot be used in occupied spaces, since light may be harmful to skin and eyes

Only practical when light can reach surfaces; many technical issues must be considered when installed

High-intensity targeted heating

Can be deployed in stand-alone units or through HVAC systems

Requires air to be exposed to heat for signi cant periods and thus may be di cult to apply when there is a continuous ow

Includes ionized puri ers and ozone generators; mechanism of action varies by technology

Virus still active

Virus is made inactive via sterilization process, which can be challenging for a continuous stream of air or aerosols

Source Expert interviews; McKinsey analysis

The mechanical filters in HVAC systems have tangled fibers that trap particles too large to fit through the openings. Mechanical filters have different ratings, based on the percentage of particles they remove, with the highest rated typically used in surgical or clean-room applications. Exhibit 2 shows selected filters and their ratings from organizations based in the United States. Ratings standards in other countries may vary.

High-Efficiency Particulate Air (HEPA) filters are most effective at removing small particles. To meet this qualification under the US ratings system, filters must remove 99.97 percent of particles of 0.3 microns. Minimum Efficiency Reporting Value (MERV) filters are assigned ratings according to their ability to filter out large particles (from 0.3 to 10.0 microns in size). MERV filters with ratings of 17 or higher are comparable to HEPA filters

and may be referred to by that term.8 Like the airconditioning systems in most homes, commercial buildings generally have filters rated MERV 12 or lower. Only some air conditioners can accommodate HEPA filters, and technicians must configure them properly and replace them regularly.

Upgrading HVAC systems by incorporating highergrade filters can be very expensive and is not always feasible. What's more, even a HEPA filter will not eliminate all concerns about airborne transmission. Although a NASA study documented that HEPA filters can stop particles as small as 0.1 microns--the approximate size of the coronavirus--other direct research is limited, and the official US ratings system specifies their efficacy only for particles of 0.3 microns.9 More research is needed to definitively determine a minimum filtration rating that will eliminate infectious coronavirus particles from air.

8 Nathan Frey, "Do HEPA air purifiers filter out the COVID-19 virus?," March 20, 2020, . 9 J. H. Agui, J. L. Perry, and R. Vijayakumar, "Submicron and nanoparticulate matter removal by HEPA-rated media filters and packed beds of

granular materials," National Aeronautics and Space Administration, May 2016, .

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Can HVAC systems help prevent the transmission of COVID-19?

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Minimum e ciency rating value? (MERV) and high-e ciency particle air? (HEPA) lters, % e ectiveness

Lower e ciency

Higher e ciency

E ciency rating

MERV 11 12

MERV 13 15

MERV 16

(N 95 respirator equivalent)

HEPA lter

(P 100 respirator equivalent)

Captures 65 80% of particles

1 3 microns in size (eg, lead dust,

vehicle emissions)

Captures 50 85% of particles 0.3 1.0 microns in size (eg, smoke, exhalation droplets)

Captures 95%

of particles 0.3 1.0 microns in size (eg, smoke, exhalation

droplets)

Captures 99.97 99.9995%

of particles 0.3 microns in size

(eg, bacteria, viruses)

Applications

Air-pressure requirement

Superior residential, better commercial

Day surgery, general surgery, smoking lounges

Low-to-moderate air pressure required

Moderate-to-heavy air pressure required

Low-level nuclear, cleanrooms, laboratories

Heavy air pressure required, using a

stronger fan

?MERV is the ASHRAE standard for a lter's particle-removal e ciency (MERV ranges from 1?20; only displaying higher-rated lters). To achieve e ciency rating, lters must be clean. Air ow capacity is a function of the resistance or pressure drop across the lter and particle loading. As dust cake forms in lter, the

resistance increases and air ow rate decreases, and the lter no longer achieves its e ciency rating. ?A MERV lter rated 17 or higher would meet HEPA standards and would typically be referred to as a HEPA lter. Source: ASHRAE; CAMFIL; EMW.DE; MT Pinnacle; NAFAHQ; US Environmental Protection Agency

Airflow management

While studies are still ongoing about how the coronavirus spreads via air, evidence suggests that measures to change indoor airflow patterns could play a role in reducing transmission. Three main principles apply:

-- encouraging a vertical laminar rather than turbulent airflow

-- ensuring a slow, steady air speed

-- directing potentially contaminated air out of rooms and away from people

In-room airflows The World Health Organization recently acknowledged that some evidence about in-room transmission is worrisome. In addition, after analyzing a transmission event at a restaurant in China, the US Centers for Disease Control and Prevention (CDC) concluded that an asymptomatic patient transmitted the virus to families at two nearby tables (Exhibit 3).10 Based on the restaurant

10Jianyun Lu et al, "COVID-19 outbreak associated with air conditioning in restaurant, Guangzhou, China, 2020," Centers for Disease Control and Prevention, early release, July 2020, .

Can HVAC systems help prevent the transmission of COVID-19?

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